We both seek to know whether or not Mode S is required for IMC flight after March 2007 as it will be for IFR
Not sure you mean what you've actually written. All flight in IMC must be under IFR, unless it is a flight under SVFR in a control zone.
In the UK, Mode S is currently required only for flight in the London TMA (which is necessarily under IFR). An exemption exists until 31 March 2007, but from that date, it should still only be flight in the LTMA that requires a Mode S transponder. Other airspace may subsequently become mandatory-Mode-S in due course, but I very much doubt that any of it will be airspace in which an IMC-rated pilot is permitted to fly under IFR (Class D/E) any time soon.
This thread has more info.